Practically every function in the human body has been shown to exhibit circadian
rhythmicity. Under controlled conditions in the laboratory, the endogenous nature of the rhythmicity can
be demonstrated. In ambulatory conditions, environmental factors and physical exertion can obscure or enhance
the expressed rhythms.
The three most commonly monitored vital signs are blood pressure (systolic and diastolic), heart rate, and
body tem- perature. Because these variables exhibit daily rhythmicity, their normal values vary with the time of day.
The table on the right shows the normative ranges for these variables.

Normative values are derived from ambulatory data
collected from healthy sedentary individuals. The ranges account not only for daily variability but also for
variability associated with sex, age, and individual differences.

Blood Pressure

Of the three major vital signs, blood pressure is the most irregular one
under ambulatory conditions. In order to obtain the figure shown on the right,
data from seven consecutive days had to be averaged. (The patient was a healthy
sedentary adult male who used to sleep from 01:00 to 09:00 each day.)

Although the data evince considerable random variability, a
daily pattern can be observed. Blood pressure falls during sleep (indicated by the
grey rectangle), rises at wake-up time, and remains relatively high for the next
6 hours or so.

This patient had a mean systolic pressure of 117 mm Hg
(diastolic pressure is not shown). Notice that values as low as 103 mm Hg are normal
during sleep and that values as high as 133 mm Hg are normal in the first part of the
day. Clearly, the concept of hypertension is dependent on the time of day, and a single
measurement taken at the doctor's office can be very misleading (although,
of course, systolic pressures above 140 mm Hg would characterize hypertension at any
time of the day under sedentary conditions).

Heart Rate

If physical exertion is avoided, the daily rhythm of heart rate is robust even under
ambulatory conditions. As a matter of fact, ambulatory conditions enhance the rhythmicity
because of the absence of physical activity during sleep time and the presence of activity during
the wakefulness hours.

The figure on the right shows the values of heart rate of an adult sedentary
male averaged over seven consecutive days. Heart rate is clearly lower during the sleep hours
than during the awake hours. The mean heart rate for this patient was 69 bpm, with
lows at 55 bpm and highs at 83 bpm.

Body Temperature

Of the three major vital signs, body temperature is the one that
has the most robust rhythm. The rhythm can be disrupted by physical exertion,
but it is very reproducible in sedentary people. The figure on the
right shows the values of oral temperature of an adult sedentary
male averaged over seven consecutive days. Notice that the curve is much
smoother than those of blood pressure and heart rate. Notice also that the
concept of fever is dependent on the time of day. This patient had a mean
tem- perature of 36.9°C, but tempe- ratures varied from 36.2°C to 37.4°C.
At 05:00, a reading as low as 36.8°C would be indicative of (mild) fever.
At 19:00, a rea- ding as high as 37.5°C would not be indicative of fever.

The material posted on this web site is based upon work supported by the National Science Foundation
and the National Institutes of Health. Opinions, findings, conclusions or recommendations expressed
in this material are those of the author and do not necessarily reflect the views of either the National
Science Foundation or the National Institutes of Health.